The conventional method of forming an intermediate layer or cover layer of a golf ball utilizes an injection mold having two mold plates with hemispherical cavities that mate to form a spherical mold cavity when the mold halves are joined. At the initial stage of the injection molding process, a golf ball core is supported centrally within the mold cavity by a plurality of retractable pins near the upper and lower poles of the mold cavity so as to leave a space around the core for forming an intermediate layer or cover layer. A thermoplastic or thermosetting material then is injected into the mold cavity near the equator of the mold. FIG. 1 illustrates a conventional golf ball injection mold cavity.
Conventional golf ball injection molds also include vents near the upper and lower poles of the mold cavity, near the last part of the mold cavity filled by injected layer material. The air and gasses inside the mold cavity normally are evacuated by natural ventilation through clearances between a vent pin 20 and the wall of a vent pin hole 22 in the mold plate. If the newly formed layer is a cover layer, the vent pin also is preferably located at a dimple location so that the pin forms the dimple.
As shown in FIGS. 2A-C, a conventional vent pin 20 may be configured with a primary vent 24 and a secondary vent 26 on the outer surface of the vent pin. The primary vent is located at the tip 28 of the vent pin that forms part of the mold cavity wall and is sufficiently narrow to prevent injected material from forming flash on the newly formed layer, while the secondary vent 26 typically has a wider and deeper opening so that trapped air and gasses are more readily released from the mold after escaping from the mold cavity through the primary vent 24.
A conventional primary vent may have a flat side formed on the edge of the tip that is wide enough to allow trapped air and gasses to escape the mold cavity but narrow enough to prevent the forming of flash on the newly formed layer. Alternatively, as shown in FIG. 2B, the outer diameter of the vent pin tip may be such that a thin circular gap 30 is created between the tip of the vent pin 28 and the wall of the pin hole 22. FIG. 2C illustrates that vent pins also may be made of porous metal having open pores that are large enough to allow air and gasses to pass through them but small enough to prevent flash from forming. Alternatively, the tip of the pin may be made of porous material in fluid connection with a secondary vent. After passing through this narrow opening or porous material at the tip of the vent pin, the air and gasses are released from the mold through the secondary vent.
In addition to ventilating air and gasses through vents in the mold, some trapped air and gasses also may escape the mold cavity through clearances between the retractable pins 32 and the retractable pin holes 34 in the mold plates 36. The release of air and gasses in this manner, however, is significantly less than the amount of air and gasses that escape through a vent because of the differences in design and positioning of a retractable pin in comparison to a vent pin. In particular, retractable pins do not have flats on the portion of the pin facing the equator of the ball, nor do they utilize a porous metal tip because doing so may cause the pins to bend or spread, thereby adversely affecting the ability of the retractable pins to accurately and securely position the ball core in the mold cavity during the injection process. Moreover, because retractable pins are located in the mold in order to securely hold the core in place, the material filling the mold cavity typically covers this portion of the mold well before the injection process is complete. Thus, air and gasses may escape through the retractable pins for only a portion of the injection process.
Despite that conventional vent pins are configured with primary and secondary vents or porous tips to increase ventilation capacity, ventilation of trapped air and gasses inside the mold often remains a limiting factor in the speed at which material is injected into the mold cavity. If the vent holes are too small, poor ventilation can cause improper or inadequate venting of trapped air and gasses from the mold cavity during injection, which can have a deleterious effect on both the visual quality and durability of the newly formed layer. If the injection speed of the material is too fast, the speed of evacuating air and gasses out of the mold cavity during the injection process can cause the newly formed layer to scorch or not completely fill the mold cavity. Conversely, if the vent holes are too large, the injected material flows thereinto and forms flash on the newly formed layer, thereby requiring substantial additional processing for removal of the flash and surface finishing.
One solution to the problem of inadequate venting is to reduce the injection speed of the material. While lengthening the time for injecting material into the mold does provide better correspondence between the rate of ventilation of air and gasses and the rate at which the mold cavity fills with layer material, the reduction of injection speed also may reduce the ability of material injected from any one injection gate to intermix and weld together with material injected from neighboring gates, which may reduce the durability of the layer and useful life of the ball. Moreover, reducing injection speed also results in lower overall production capacity due to the increased injection time.
The use of porous material as a vent presents additional disadvantages. For instance, the pores eventually become blocked by contaminates over time through use, thereby reducing the capacity of the vent trapped air and gasses. The diminishing ventilation capacity may require corresponding reductions of the injection speed of the material to avoid the problems that may result from inadequate ventilation. These frequent adjustments can be time consuming and significantly reduce ball manufacturing capacity due to increased downtime. Moreover, cleaning the porous metal to remove contaminates blocking the pores also can be time-consuming and expensive. When the porous metal becomes overly blocked by contaminates, the metal must be cleaned with a solvent ultrasonic bath or by controlled pyrolosis methods such as a burn off oven or fluidized bed.
Vents also may be used to assist in the ejection of the ball from the mold after completing the injection molding cycle. Typically, the retractable pins of the golf ball mold are struck by an ejector bar that causes the retractable pin tips to strike the surface of the ball and eject it from the mold cavity. If the ejection force imparted on the retractable pins is too high, the faces of the pins may deform the newly formed layer as they strike the surface of the ball. In addition, the ejection force imparted on the retractable pins may cause the pins to bend or spread over time, particularly if the ejection force imparted on the pins is high. Both of these problems are alleviated by forcing air through the vents into the mold cavity during ejection in order to reduce the amount of ejection force needed from the retractable pins to remove the ball from the mold. Increasing air blow volume through the vents would further alleviate these potential problems by further reducing the amount of ejection force needed from the retractable pins to remove the ball.